Complaint AC unit running for hours

Why would it be illegal exactly? It's outdoor noise. Just happens to be coming inside. My suggestion is to find a way to quiet the noise in your home. Put up some solid shades or thick curtains. Get a sleep machine to drown out the noise. Wear earplugs. Stop letting it consume you.

Contact the HOA board and see if they can enforce a noise restriction. Otherwise, it's going to be an annoyance. Believe me, I worked 3rd shift for several years. My neighbors I swear would mow their lawn intentionally outside my window. Heck, even when it rains these people will mow! I ended up with a sleep machine, ear plugs, and an eye mask.

Adding more useful info:

Hopefully someone from California will chime in, but for now I'd suggest Googling local noise ordinances for the area as well as laws governing AC units.

Condo associations don't enforce local ordinances. What they do enforce are restrictions in the governing documents of that community. So check to see if there is any language that addresses HVAC issues and maintenance requirements. Everyone living around the noisy AC unit should be hearing the same thing. Has anyone else complained? If not, then this becomes a neighbor-to-neighbor dispute, and boards generally stay out of these unless one of the neighbors is doing something that violates the governing documents.

The trouble with noise complaints is that there is no objective standard for what is reasonable, and that makes it difficult to enforce any restriction that addresses noise. In addition, some people are going to be bothered by levels that are "reasonable" for anyone else. When we were dealing with a noise complaint in my community, our attorney noted that people need to choose housing situations that meet their needs, and those who are sensitive to noise should not expect that a community is going to adjust itself to meet their preferences. This means looking at things like noise canceling head phones.